Purely Medicinal
by Kate Matty
Summary: Most men do it for fun. For Sherlock, it's purely medicinal.


"Did you know that regular masturbation decreases the risk of prostate cancer?"

John had been living with Sherlock long enough for questions like this to have become the norm. As such, John only let his jaw drop for a fraction of a second before resuming his entry into the flat and proceeding to put away the groceries.

"I did, as a matter of fact." John said. "I _am_ a doctor, you know."

Sherlock looked up from John's laptop and fixed his flatmate with a look of indecent outrage. "You _knew _about this?"

John was rather taken aback by the accusatory tone. "Well, yes. Lots of people know that. Well, a few. What difference does it make?"

Sherlock rolled his eyes with such force it was a miracle the tendons didn't snap. "Oh, none at all John. What difference could it _possibly _make to impart vital information that has the potential to save lives. Do you not realise what this means?"

John scratched his chin absently as he made tea. "That a wank a day keeps the doctor away?" he tried jokingly.

Sherlock scowled, resenting John's humour in what he considered to be a wholly un-funny situation. "Brilliant deduction, Dr Watson. More to the point, the _absence _of a wank a day may result in early and _excruciating death._"

John nodded. "Yes, that would make a much catchier slogan for prostate cancer awareness."

Sherlock was now fidgeting with his forearm, a sign that he was in need of a nicotine fix. _He's worried, _John deduced, cursing that Sherlock's observational habits were rubbing off on him. "The numbers on this are all over the place… I can't find any accurate information about how this affects _me…" _Sherlock muttered, looking more and more agitated by the second. "John, in your medical opinion, how long do I have left?"

"Oh for…" John was trying very hard not to laugh, as his flatmate seemed to be in real distress. "Sherlock, you're not even forty. You're not going to get prostate cancer at your age. Yes, it probably would have been healthier if you'd… cleaned the pipes every now and again. But I seriously doubt you're currently in any mortal peril."

"What if I started making up for lost time?" Sherlock continued, his face deadpan. "If I calculated correctly, I could masturbate ten times a day, and by that time I should be caught up by…"

"It doesn't work that way, Sherlock." John rubbed his temples with his thumb and forefinger. He'd been in the flat less than five minutes and was already getting a Sherlock-induced migraine. That had to be a new record. "Look, if it's bothering you that much, just make a habit of it now. I'm sure that will be enough to keep the cancer at bay."

Sherlock nodded thoughtfully. Then without any hesitation or embarrassment, he sprang up from the couch and made a beeline for the bathroom.

John tried very hard not to think about what he knew for certain Sherlock was doing in there.

.

.

In retrospect, John probably should have thought about the consequences of advising an idiosyncratic eccentric to add a daily masturbation session to his routine.

John's first insight to this came the next morning, as John was tiredly stumbling to the half-open bathroom door, only to step inside and find Sherlock Holmes stroking himself lazily over the bathroom sink.

"Morning, John." He said airily as his roommate alternated between wide-eyed staring, and screwing up his eyes in horror.

"Sh-sherlock?" John spluttered. "What are you _doing?"_

"What does it look like?" Sherlock said derisively. To John's irritation, he didn't even pause in his movements, but continued massaging his shaft with languid, unhurried strokes.

"It _looks _like you're having a morning wank with the bloody door open!" John finally settled on glaring angrily at the patch of wall behind Sherlock, straining to focus on the peeling wallpaper and _not _on his flatmate's highly-inappropriate morning activities.

"Yet another brilliant deduction. You're getting better and better at this." Sherlock drawled, though his usual air of superiority was somewhat ruined by the little shudder he gave as his cock suddenly became much more sensitive and responsive. _What could that be due to? _Sherlock made a mental note to wonder about that later.

"Yes but… stop that! You left the door open!"

"So? I always leave the bathroom door open. You've never complained when you walk in on me showering, or brushing my teeth or taking a piss…"

"That's… that's different." John argued lamely. It was true, they hadn't really squabbled over bathroom etiquette. Being men, they didn't seem to mind walking in on each other changing or showering or taking a leak. John attributed it to being in the army, where privacy was scarce. He'd attributed Sherlock to… well, being Sherlock.

"Why is this different?" John wished Sherlock hadn't nodded towards his penis as he'd said that, causing John to involuntarily look down again.

"It just _is._" John said firmly, giving up on averting his eyes so he could glare at Sherlock. "I can't brush my teeth with your dick in the sink. Can't you do that in your bedroom?"

"Yes." Sherlock answered. "I can also technically brush my teeth, bathe, and empty my bowels in my bedroom, but these things tend to be less messy when conducted in the bathroom. Can't you wait til I'm done?" Sherlock asked, having the nerve to sound politely affronted by John's lack of manners.

"I need to brush my teeth!"

"I need to fight off cancer!" Sherlock's cheeks had a definite pink tinge now, but it was hard to tell if it was from anger or from… John bit his lip to stifle a moan. Jesus, was he dripping?

"Honestly John, you're being very selfish."

"_I'm_ selfish?" John asked, his words catching on a laugh. "I'm not the one making my roommate late for work so I can indulge in a morning wank!"

"I'm not the one willing to let my roommate get cancer so I won't be late to work." Sherlock countered. "And I'm not _indulging _myself. This is a matter of life and death, John."

John snapped. "Sherlock, if you don't put yourself away and let me brush my teeth _right now, _I'm going to come over there and…"

But John didn't have time to think of what he was going to do, because as soon as he'd started shouting, Sherlock's hand had started flying madly over his prick and the next moment, Sherlock's entire body had started to twitch. Sherlock's free hand gripped the rim of the sink, and the bathroom rumbled with a low moan that somehow resonated from Sherlock's chest but never made it past his lips. John watched with fascinated horror as Sherlock's eyes closed in bliss as he ejaculated into the bathroom sink, thrusting into the tight circle of his fist and shooting nearly all of it neatly down the drain.

"That…" Sherlock said, sounding slightly out of breath. "Won't be necessary."

John watched, stunned, as Sherlock washed his hands, used a bit of liquid soap and toilet paper to wipe the basin and _finally _tucked himself away.

"All yours, John." Sherlock smiled, clapping John cheerfully on the shoulder as he left.

Sherlock wasn't the only one to fill the basin that morning.

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**Review if you enjoyed!**

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**(I generally don't both continuing fics if I think no one is reading them, so if you want more, plz review)

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Also, I just like having my ego stroked. Who doesn't?


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